The instrument has an elongated shaft that is extended away from housing, and a branch (19) with an electrode (33). A branch (20) is supported to movable toward and away from the electrode, where the branch (20) has a longitudinal slit (31). A knife (21) is arranged to movable with a longitudinal movement along a shaft and arranged to extend into or movable into the longitudinal slit. The knife is associated with a vertical adjustment mechanism for adjusting the knife perpendicularly to the longitudinal movement as a function of the longitudinal movement of the knife. An independent claim is included for a method for operation of instrument.
